That's exactly where I was 17 years ago - planning a cruise where I wanted more house capacity than my 2 gp 27s could give me (keeping one in reserve). The light came on and I realized I could double my house capacity by adding a gp 24 for the engine.
Of course, you'll still need a combiner. I also recommend getting rid of the A/B switch and go with ON/OFF switches to avoid confusion. For now, you can keep the A/B for the engine, but get an ON/OFF for the house bank keep them in parallel.